Platinum-coated Nafion 117 structures were characterised using electrochemical measurements of platinum surface area and a number of microscopy techniques. The morphology and composition of the platinum deposits were related to their preparation conditions in terms of platinum salt concentration, electrolyte flow and the surface roughness of Nafion 117. Platinum surface areas achieved were higher than the values predicted for ideal spherical platinum particles of average diameter. This is due to a fine microstructure, which realises much smaller platinum particles than average (down to about 50 nm) allied to the geometry produced by their clustering to form nodules (about 0.1-1.5 mm diameter micro-nodules and about 3-5 mm macro-nodules). Adherent platinum deposits with high surface areas were promoted by using roughened Nafion 117 membranes and enhancing the mass transport of chloroplatinate and tetraborohydrate ions by magnetic stirring of the electrolyte. A flow cell produced much more reproducible platinum-coated Nafion 117 structures. At best, platinum surface areas of 30-50 m 2 g 21 Pt were achieved at platinum penetration depths of 5-30 mm into the membrane surface.

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